1. Field of the Invention
The invention relates to MEMs gyros and in particular to a method of suppressing errors.
2. Description of the Prior Art
As MEMS inertial sensors have begun to proliferate more into rate and tactical grade application markets, the current demand is for inertial sensors with higher precision and long term performance. Currently, there are no MEMS based gyroscopes on the market capable of navigation grade inertial sensing, mainly due to inadequate drift and noise performance that result in large attitude errors upon integration of rate signals to obtain orientation. One of the contributing factors to this degradation in gyroscope performance is structural imperfections as a result of tolerance errors in the fabrication of the device. As all MEMS devices are built using photolithographic processes, the relative tolerances are on the order of 10% or more. Currently, in order to operate with the highest precision, vibratory gyroscopes typically include active feedback control to compensate for fabrication imperfections. However, as will be illustrated in this disclosure, when imperfections are large compared to the measured Coriolis force, compensation cannot be achieved with a purely feedback control without interfering with the Coriolis measurements. These interferences cause scale factor and drift errors in the gyroscope, resulting in degraded performance. In these cases, both post processing such as laser trimming, ion beam milling or selective material deposition and feedback control are required. The disadvantage is that this type of post processing is done exclusively by the manufacturer, usually under ideal laboratory conditions. As a result, the end-user is still required to calibrate the device prior to use and then once the device is in use in the end-user's application, typically no additional calibration is possible.
With the continuing improvements of CMOS compatible MEMS processes, the prospect of enhanced microdevices capable of computationally intensive integrated control systems is fast becoming a reality. In light of this fact, the demand for improved inertial sensor performance gives rise to a new paradigm of “smart” devices with enhanced capabilities, such as active structural compensation, self-calibration, and signal processing integrated on the same chip. Under this new paradigm, what is needed is an alternative to the potentially costly and time consuming post processing of each individual device.